<!DOCTYPE html>
<html>
    <head>
        <title>Example</title>
            <!-- 
        可以以calc()算式为值
         -->
    <style>
        p {
            background: grey;
            color: white;
            font-size: 20pt;
            width: calc(80% - 20px);
        }
    </style>
    </head>

    <body>
        <a href="http://apress.com">
            Visit the Apress website
        </a>
        <p>I like <span>apples</span> and oranges.</p>
        <a href="http://w3c.org">Visit the W3C website</a>
    </body>
</html>